1. Field of the Invention
The present invention relates to an OFDMA (Orthogonal Frequency Division Multiplexing Access) system, and more particularly to an apparatus and method for minimizing interference caused by sub-channel frequency overlaps when assigning sub-channels for sectors of a base station in an OFDMA system of a FDMA (Frequency Division Multiple Access) type.
2. Background of the Related Art
Recent advances in mobile communication technologies have led to a move from using analog methods to digital methods in signal processing. Also, in addition to voice and text communication services, multimedia communication services are now also provided for commercial use. In line with such progress, there has been a growing interest in the 4th generation communication system, which establishes a more effective communication environment based on systematic inter-operations between a mobile communication system and a radio access and wide-band wired system.
Also, an OFDM transmission method is being proposed as a radio transmission method in order to enable various services to be provided in the 4th generation communication system. The basic principle of the OFDM transmission method is to transform data rows inputted in series into a plurality of parallel data rows so as to separately carry each of the parallel data on a separate sub-carrier. In such a case, the frequency of the sub-carriers must be selected to maintain orthogonality between the sub-carriers, wherein such orthogonality allows each of the sub-carriers to overlap in the spectrum region.
Also, as for the 4th generation communication system based on the prior OFDM transmission method, an OFDMA technology based on the OFDM system has been proposed as a multiple user access method in order to satisfy various QoS (Quality of Service) requirements, i.e., to maintain service quality while supporting various types of services and applications. The OFDMA method has the advantage of providing frequency and time in accordance with the user's requests by dividing the frequency band and time.
Further, since the same medium (channel) is shared by a plurality of users, the interference from neighboring cells or sectors greatly affects the communication quality. Thus, in order to reduce the interference, a FDMA based system uses a frequency reuse factor of 1/7 as a default value, while a CDMA based system employs power control methods, for example. In contrast, the OFDMA system aims to use 1 as its frequency reuse factor. However, this inevitably leads to the interference phenomenon caused by the sub-carrier frequency overlap when there is an increase in the number of users in the multiple-sector environment.
That is, in the multiple sector (or cell) environment exemplified in FIG. 1, the sub-carriers will inevitably be assigned to multiple sectors or cells when a sum of cell loads of each sector constituting the base station is greater than 1. This requires a resource assignment method, which is different from the prior methods, in order to reduce the interference effect of these multiply assigned sub-carriers.
Also, prior OFDMA systems assign resources by unit of sub-channels consisting of a combination of sub-carriers and can be classified as PUSC (Partially Used Sub Carrier), FUSC (Fully Used Sub Carrier) or Band AMC (Band Adaptive Modulation Coding) types, etc. according to their sub-channel combination type. The PUSC and FUSC types are designed to avoid the same sub-carriers from being in proximity with each other when preparing the sub-channels to obtain frequency diversity. The Band AMC type is designed to allow the sub-carriers to be in proximity with each other when preparing the sub-channels to maximize the utilization of a change in the channel. However, conventionally, the assignment of the combined sub-channels is not managed by a single base station or controller, but rather independently by each sector, such that the problem of the interference caused by frequency overlap (i.e., sub-carrier overlap) arises.
FIG. 2 is a diagram illustrating a case of sub-channels randomly assigned in a conventional OFDMA system. The first sub-channel assignment method illustrated in FIG. 2 is a conventional random assignment method. When the total sum of the cell loads of each sector is greater than 1 (½+½+½=1.5), not only are there sub-channels of class 0 which are not assigned to any of the sectors (channel indexes 43-48), but there are also a lot of sub-channels that are inefficiently assigned. This increases the number of sub-channels of class 2 (a group of sub-channels assigned to 2 sectors, channel indexes 7-18, 25-30) and the number of sub-channels of class 3 (a group of sub-channels assigned to all sectors, channels indexes 1-6). Thus, the interference phenomenon caused by the frequency overlap cannot be efficiently managed.